The present invention relates toga method for applying a layer to a piston ring blank, in particular for a piston ring of the kind that is intended for use in large diesel engines.
The invention also relates to a device for performing the method.
Piston rings of the kind which are preferably used in large diesel engines, for instance in boat engines, are exposed to extreme conditions during the operation of the engine. A high degree of wear, heat and corrosive attack cause the piston rings to quickly wear, thus necessitating replacement.
Many methods for increasing the service life of the piston rings have been proposed. Some of these methods are directed to the application of an abrasion-resistant layer to the most exposed portions of the piston ring, preferably its outer periphery. These wearing layers can, for instance, be applied chemically to the ring material by means of an electrolytic reaction or be welded to the same under the action of a heating means, often an energy beam.
Today a plurality of methods are available for coating a workpiece by means of an energy beam, for instance a laser beam, which heats a zone of the workpiece. In such coating, a coating powder is usually melted together with a workpiece, resulting in both the powder and some of the material of the workpiece being melted. One draw back of this method is that the material of the workpiece is subject to changes which are not desirable, such as the formation of martensite. If martensite forms under the coating layer, there is a great risk that the layer will disengage from the workpiece during use.
Another drawback of such coating is that it requires different stages of heating and cooling of the workpiece. Stresses may thus arise both in the workpiece and between the workpiece and the coating layer and affect the strength of the coating and the workpiece.
Moreover, when manufacturing piston rings, specific problems arise. A piston ring is usually formed by cutting xe2x80x9cslicesxe2x80x9d from a cast pipe which is somewhat oval in cross-section. In one processing step, the oval rings are formed with a through slit to allow the passage of gases as well as satisfactory contact with the lining of the piston when operating the engine. The oval shape as such is difficult to handle, compared with more symmetrical shapes. The necessary stress in the material is critical to the function of the piston ring but difficult to combine with a strong, adequately attached wearing layer in the form of a coating.
The present invention aims at providing a method and a device for applying an abrasion-resistant and well-attached wearing layer to a piston ring blank.
The object of the present invention is achieved by means of a method according to the introductory part, wherein a coating powder is supplied to one of succeeding portions of the piston ring blank, after which the coating powder is melted with the aid of an energy radiation means to form a coating layer on said portion of the piston ring blank.
The energy beam is allowed to act on the coating powder, its energy being absorbed by the powder and converted into heat, which melts the powder. By using correctly chosen energy radiation, the powder is melted with a minimum effect on the piston ring blank. This melting is just enough for satisfactory welding of the coating layer and thus allows a minimisation of the Heat Affected Zone (HAZ) in the piston ring blank. This finally results in a piston ring having a strong coating layer which is firmly bound to the piston ring.
Preferably, the energy radiation is allowed to act on the coating powder for a period of time which is adjusted so that substantially no heat, besides the heat required for melting the powder material, develops. The energy radiation can be adjusted to the composition of the coating powder and the amount of powder, in which case the method can be controlled by the time of action, so that substantially only the amount of heat required for melting the powder material develops.
The energy radiation which is used to melt the powder can, preferably, be laser radiation, but also for instance plasma radiation is feasible.
According to the second aspect of the present invention, the object is achieved by means of a device for applying a layer to a piston ring blank, in particular of the kind that is intended for use in large diesel engines. The device comprises a powder feeding means, which is arranged in the vicinity of the piston ring blank for supplying a coating powder to one of succeeding portions of the piston ring blank, and an energy radiation means, which is arranged in the vicinity of the piston ring blank for melting the coating powder in said portion.
The device is preferably adapted so that the energy radiation is allowed to act on the coating powder for a period of time which is adjusted so that substantially no heat, besides the heat required for melting the powder material, develops.
The energy radiation means is preferably a laser means for producing a laser beam. This type of means is well known and relatively easy to control.
According to the method, the piston ring blank is moved preferably successively relative to the powder feeding means and the energy radiation device. After each portion of the piston ring blank, a new portion of the piston ring blank is thus successively advanced, which allows the method to be performed continuously.
Consequently, the device according to the invention is preferably provided with a driving means for relative movement between the piston ring blank and the powder feeding means as well as the energy radiation means. The driving means is then suitably arranged so that the portion of the piston ring blank first passes the powder feeding device and subsequently the energy radiation means. According to the method, the powder is applied before heating by means of the energy beam. As a result, the piston ring material is not heated more than necessary to form a coating layer by melting the coating powder. This minimum heating of the piston ring material is, as already mentioned above, the reason why the heat affected zone (HAZ) and thus the formation of martensite in the piston ring blank is minimised.
The method is suitably initiated and finished in one and the same zone of the piston ring blank, because the starting and stopping zone risks becoming uneven or irregular. By locating these zones adjacent to each other, it is possible, when manufacturing the piston ring of the piston ring blank, to choose precisely this zone to form the necessary slit. The material that is used in the finished piston ring will thus be free from stress or irregularities in the edges of the coating.
The piston ring blank is preferably rotated substantially on its own axis as the method proceeds. Suitably, the driving means can be arranged to rotate the piston ring blank substantially on its own axis. In such an embodiment, the powder feeding means and the laser device can both be arranged in a stationary manner, the piston ring blank being fed successively past them.
The energy radiation is preferably allowed to act at a constant distance relative to the coating powder which was last supplied to the piston ring blank. This results in an even energy dose which is converted into heat in the coating powder. Thus the quality of the coating layer will be even along the entire piston ring.
Preferably, the width of the energy radiation substantially corresponds to the width of the piston ring blank, and in the device the energy radiation means is correspondingly adapted to generate a laser beam whose width substantially corresponds to that of the piston ring blank. A wide energy beam during the coating method contributes to an even quality of the coating layer in the centre as well as at the edges of the piston ring portion.
The coating powder is preferably chosen so as to be of a kind having substantially the same coefficient of thermal expansion as the piston ring blank, which prevents the emergence of stress in the materials when heating and cooling. To obtain the required strength, the powder material can suitably be of a kind that forms a metal matrix comprising Cr3C2 and Mo. Suitably the powder material then contains 30-50% by weight Cr, 5-15% by weight Mo, 1.5-5% by weight C, 30-50% by weight Ni and 1.5-5% by weight W.
Suitably, the piston ring portion is pre- and post-heated by means of a plurality of heating elements. The pre- and post-heating of the blank serves to reduce the stress in the two materials resulting from excessive and rapid temperature changes. The advantage of arranging a plurality of heating elements is, among other things, that they can be controlled individually, and consequently the heating can be well controlled. In addition, this heating takes place as the method proceeds.
These heating elements are preferably evenly distributed around the circumference of the piston ring blank, which minimises the stress in the ring.
Preferably the heating elements can consist of induction coils. The use of such coils is particularly advantageous as the surface of the piston ring blank is then heated to a greater extent than its interior. As a result, the piston ring blank is not heated more than necessary for the actual coating, and thus the formation of martensite in the material is reduced.
The device preferably comprises a suspension device for the piston ring during the coating method. The suspension device comprises at least one supporting means against which a zone of the piston ring blank abuts.
Advantageously, the suspension device can be a device in which the piston ring is caused to rotate. When the piston ring blank is somewhat oval, it is particularly suitable to use a device comprising spring-loaded supporting means, which act radially outwards to the inside of the piston ring blank. Owing to this, the supporting means follows the oval shape without any difficulties. The supporting means and the heating elements can easily be arranged at the circumference of the piston ring blank without interfering with each other, for instance, in a manner such as described in the embodiment below.